The prevailing wisdom at the time of the first anthrax exposure was that one needs to inhale at least 8,000 to 10,000 spores before it become fatal. The 8,000 to 10,000 spore number came from the recommendations of the Center for Disease Control and Prevention (CDC) in the United States. CDC also suggests that for cutaneous anthrax, a victim need to come in contact with hundreds of spores.

Since it is highly unlikely that people can get exposed such a large number of spores by incidental contact like cross contamination from mailed letters, it was widely assumed that the danger is limited to those who have been directly exposed.

However, this theory seems to have some holes. Experts question where there is any "safe exposure limit" for anthrax.

We had been pretty well limited to our direct understanding of the effect of anthrax contamination. Till the terrorist action, most of the experience comes from the
exposure through animals.

Dr. Matthew Meselson, a Nobel Prize-winning molecular biologist at Harvard University argues that the threshold theory of anthrax exposure being used now by CDC and other federal health officials is based on U.S. Army studies performed decades ago on monkeys that never reached consistent results.
The range of spore doses necessary to kill half the exposed animals in those studies was from 2,000 to 50,000, varying according to how the experiments were conducted and what type of anthrax was used.

However, none of the primate studies used dried powder forms of anthrax, such as have been mailed to various destinations in the United States in recent weeks. The terrorist
manufactured anthrax is very fine and has been processed to prevent it from clustering together and dispersing harmlessly. Obviously, we are talking of a different "ball game" here than what was tested on the monkeys decades ago.

Dr. Meselson knows what he is talking about. He is the leading American expert on the only large-scale release of anthrax powder ever recorded. According to him, some who were downwind of a 1979 accident at a Russian
bio-weapons plant died after inhaling as few as nine bacteria spores.

And the studies of the accident determined that those who were exposed were still becoming ill as long as 60 days after it occurred, demonstrating the bacteria's long incubation time.

The envelope mailed to Senate Majority Leader Tom Daschle's office on Oct. 15 contained 2 grams of pure anthrax. Two grams is just a little less than a teaspoon of granulated material. If it were pure anthrax, it would amount to about 20 billion spores,
according to Dr. David Sullivan, an anthrax expert at Johns Hopkins University.

In the April 2, 1979 accident at the Biopreparat weapons facility in Sverdlovsk, Russia, hundreds of people were exposed to high-grade anthrax powder. It sickened 88 people and killed 68. Interestingly, the Russian accident involved a total release of less than 1 gram of spores, dispersed over a distance of four kilometers!

Dr. Meselson's 1992 study determined that there was no threshold dose required to cause inhalation anthrax. This certainly is some cause for concern. It also suggests that cleanup efforts at contaminated facilities might be extremely difficult.

Bottom Line: If you suspect that you may
have come across anthrax, do not take any chances. There is nothing like
a "safe exposure" for today's fine-crafted anthrax. It could
be deadly.